An image field is a rectangular area on a reticle or photomask that has the purpose of containing pattern which is exposed in one “shot,” “flash,” or “scan” in the lithographic process with actinic radiation, thereby leading to the creation of the corresponding pattern in the illuminated exposure field on the processed wafer. Each image field can contain several dies, i.e. pattern-filled areas that correspond to separate chips created on the wafer. The most common type of productive reticles have only one image field which often contains several identical dies with pattern for a specific chip layer. Generally, reticles with one image field will be called single layer reticles in the following. Reticles with more than one image field with pattern for different layers are commonly called multilayer reticles (MLRs).
MLRs and cross-technology reticles (CTRs) are used to reduce reticle costs, which are a large part of the total production cost in advanced semiconductor nodes. MLRs enable different layers of the same technology node to be placed on the same reticle, which reduces the number of masks to be fabricated and, therefore, the cost of a reticle set. In addition, mask processes for both a technology node, for example the 45 nm technology node, and also the next half-node, e.g. the 40 nm technology node, are often used together. In this case, CTRs containing image fields of different technologies can be used. Thus, different technology nodes and multiple layers can be present on a single mask.
CTR or MLR pairing is limited by the differences of reticle transmission (RT) between the image fields, which cannot be greater than about 25%. RT differences greater than the recommended value will lead to the degradation of mask uniformity, mean-to-target (MTT), and registration performance during mask fabrication. For advanced technology nodes, such as 28 nm, MTT and registration for image fields in the reticle are critical, and any performance degradation will affect the final yield results. Additionally, empty fields sometimes exist on critical MLRs or CTRs, which will affect the critical dimension uniformity (CDU) and registration performance of other (functional) image fields. By increasing the number of image fields within a reticle, the risk of large RT value differences will be more challenging for both CTRs and MLRs. Further difficulties arise from the pattern density of different layers, and thus the optimum pairing of the images fields, particularly for implant layers, being difficult to predict early in the reticle tape out process. Therefore, incorrect pairings of image fields may occur, especially for CTRs.
When different layers are printed on a wafer, overlay, or the relative alignment of the images on the wafer, becomes an issue. Overlay is also very important for technology nodes below the 45 nm technology node. Image field layout, i.e. the size of image fields, their arrangement on the reticle, and the choice of image field for the different layers in the production flow (or placement of layers), can be optimized to improve overlay on the wafer. One aspect is that the difference in distortion between the left side and the right side of the scanner lens will induce additional overlay errors between layers placed on different sides of the reticle. This can be avoided by restricting the layout to a multiple row layout for 45 nm technology and below MLR or CTR pairing, as illustrated in FIG. 4. The number of image fields is decided by the data size of the customer and tape out center data handling. Note that in the common contemporary scanner types, image fields are always vertically centered to the lens during wafer exposure. Therefore possible lens distortion effects are of no concern for the vertical placement of the image fields. Another aspect is the thermal expansion of the reticle during wafer exposure by absorption of actinic radiation (reticle heating), which will affect image fields at different positions on the reticle differently. This aspect is relevant for the image field placement in both directions.
For the reduction of overlay errors from both the reticle e-beam or laser writer (i.e. reticle registration) and the wafer exposure tool or scanner, the most critical layers (especially poly, contact, and first metal) should be in the same image field of the MLR layout. FIG. 5 demonstrates that if the poly layer is in the first image field of the MLR, both the contact and metal layer also should be in the first image field of their respective reticles to reduce the overlay of poly to contact and contact to first metal. This method can be extended so that poly, contact, first metal, first via, and second metal layers are all in the same image field. If the active layer cannot be paired with the poly layer, the active layer also should be put in the first image field to improve overlay. However, strict application of this rule can lead to additional empty fields (e.g., by separating active and poly layers even if they could be paired), and thus a higher number of reticles, which in turn increases cost.
It is expected that reflective reticles, particularly EUV reticles, with more than one image field will face the same challenges as conventional (transmissive) MLRs or CTRs. This disclosure therefore applies to reflective reticles in the same way as to transmissive reticles, the only difference being that for the former, reticle transmission is to be replaced by reticle reflection. The acronym RT is therefore to be understood as meaning reticle transmission for transmissive reticles and reticle reflection for reflective reticles, respectively, throughout this disclosure
A need therefore exists for methodology enabling a reduction in RT differences between image fields and optimization of layer placement for overlay without compromising cost.